of energy (potential or heat energy) gained by another aspect of the reaction (because of the law of conservation of energy). As mentioned earlier in this course, chemical reactions can be exothermic and will give off heat, or endothermic and will store or take on energy. Some chemical reactions can be written in such a way as to account for the heat given off by stating that there is heat as part of the equation. A thermochemical equation is one that has heat put into it or given off as a byproduct of the reaction. An example of an endothermic reaction is that of turning solid H2O to liquid H2O. This reaction requires the input of heat and is said to be endothermic.
LAWS OF THERMODYNAMICS There are three laws of thermodynamics that apply to all physical and chemical systems. The first law has been described, which is that energy cannot be created or destroyed. In chemistry, it also involves the concept that in all systems, the tendency of all chemical (and nonchemical) systems is to move toward the state of the lowest possible energy (which in chemistry, is the most stable system). This means that potential energy is more likely to become kinetic energy. In biochemical systems, catabolic reactions are those that take a bigger molecule (which stores energy in its chemical bonding) to be metabolized into smaller molecules plus CO2 and water. Because these are biological systems, however, the energy is given off as heat to a mild degree but goes into creating high-energy molecules like adenosine triphosphate or ATP, so that the energy is not destroyed or created but is transferred into another form of potential energy, although these molecules, too, will go on to become lower energy molecules. Even the building of molecules like glucose from CO2 and water, which are endothermic, requires the input of energy from the universe in the form of photons of light energy from the sun to make photosynthetic organisms build glucose and other molecules. In such cases, this is an open system because it requires the input of energy (photons) from the universe. In such cases, energy is not created because energy from the universe is necessary.
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